1. Field of the Invention
This invention relates to a compact DC motor incorporated with a speed control circuit, suitable for driving a capstan of a cassette tape recorder or the like.
2. Related Background Art
Motors for use in cassette recorders with radio sets and car cassette recorders are often used at a very wide range of environmental temperatures from -10.degree. C. to 70.degree. C.
In such cases, particularly in a motor of the type containing therein a speed control circuit designed to hold the speed constant by an electronic circuit, the thermal influence to which a speed control system is subjected due to an increase in the heat generated from an armature coil by driving the motor, the heat generated by supplying electric power to an electronic part such as a control IC or a control transistor, and further the heat generated in the armature coil by an electric current flowing to the armature coil being increased by an increase in the load of the instrument used caused by a change in the environment of use, and an increase in the amount of generated heat resulting from an increase in the electric current flowing into the electronic part such as the control IC or the control transistor, is great. Therefore, it has sometimes been the case that aggravation of the time drift of the rotational speed, aggravation of the rotational speed versus temperature characteristic, etc. occur.
FIG. 9 of the accompanying drawings is a longitudinal cross-sectional view illustrating a motor of this type according to the prior art. FIG. 10 of the accompanying drawings is a fragmentary perspective view showing the motor of FIG. 9 with a cap removed therefrom.
In FIGS. 9 and 10, in the prior-art compact DC motor with a speed control circuit, an electronic part 2 such as a control IC or a control transistor is directly secured to a circuit substrate 1 as by soldering, and the heat generated by supplying electric power to the electrical part warms the circuit substrate 1 directly and therefore, other electric parts mounted on the circuit substrate 1 are also warmed more than necessary. This has sometimes led to a result that the normal step of a speed control system changes to fluctuate the rotational speed and thereby reduce the accuracy of speed control.
Also, a power supply terminal has been provided on a lug 1a provided by protruding a part of the circuit substrate 1 radially thereof, and this has also led to the problem that the circuit substrate 1 protrudes from the outer diameter of a motor case 8 and the apparent outer diameter of the motor becomes so great that the mounting of the motor to a compact instrument becomes difficult.
FIG. 11 of the accompanying drawings is a longitudinal cross-sectional view showing another example of the structure of the prior-art compact DC motor. FIG. 12 is a fragmentary perspective view showing the motor of FIG. 11 with a cap removed therefrom.
In FIGS. 11 and 12, an electronic part 2 such as a control IC or a control transistor which is a part of a heat generating source in the motor is led out of the motor and is mounted on the protruding portion 1a of a circuit substrate 1 in intimate contact with the outer surface of a motor case 8.
In this structure, it is necessary to form a planar portion 8a on the motor case 8 in order to bring the electronic part 2 into intimate contact with the motor case 8 and therefore, the working of the motor case 8 which is a press-molded article has become difficult and moreover, a problem has sometimes arisen in the maintenance and checking of the press metal mold.
Again in this case, the lug 1a of the circuit substrate 1 protrudes from the outer diameter of the motor, and this has led to the problem that the apparent outer diameter of the motor becomes great and the mounting of the motor to a compact instrument becomes difficult.
Further, the protruding portion 1a of the circuit substrate 1 is relatively large, and this has also led to the problem that this protruding portion is liable to be broken or otherwise damaged.